1. Field of the Invention
The present invention relates to a continuous casting method and apparatus for effecting flow control of molten steel using a magnetic field during continuous casting of steel.
2. Description of Related Art
In continuous casting, an immersion nozzle is often used to pour a molten metal into a casting mold. If the flow speed of the surface molten metal is too high at that time, mold flux on the surface of the molten metal is entrained (or involved) into a body of the molten metal, and if the flow speed of the surface molten metal is too low, the molten metal stagnates and segregates there, thus finally giving rise to surface segregation. For reducing such surface defects, there is known a method of applying a static magnetic field and/or a moving magnetic field (AC moving magnetic field) to the molten metal in the mold for controlling the flow speed of the molten metal.
However, the known method has problems as follows. When a static magnetic field is applied to brake a flow of the molten metal (for electromagnetic braking), segregation tends to occur readily, particularly in a position where the molten metal stagnates. Also, when a moving magnetic field is applied to agitate the molten metal (for electromagnetic agitation), entrainment of the mold flux (flux entrainment) tends to occur readily in a position where the flow speed of the molten metal is high.
To cope with the above problems, several proposals have been made as to the manner of applying a magnetic field. For example, Japanese Unexamined Patent Application Publication No. 9-182941 discloses a method of periodically reversing the direction, in which a molten metal is agitated by a moving magnetic field, to prevent inclusions from diffusing downward from an agitation area. Japanese Unexamined Patent Application Publication No. 8-187563 discloses a method of preventing a breakout by changing the magnitude of a high-frequency electromagnetic force depending on vibration of a casting mold. Japanese Unexamined Patent Application Publication No. 8-267197 discloses a method of preventing inclusion defects by providing a gradient to a change rate of the magnetic flux density in the changeover process of an electromagnetic braking force so as to reduce changes of a molten metal flow. Furthermore, Japanese Unexamined Patent Application Publication No. 8-155605 discloses a method of applying a horizontally moving magnetic field at frequency of 10-1000 Hz through conductive layers, each of which has low electrical conductivity and is formed to extend continuously in the direction of transverse width of a casting mold, and imposing a pinching force on a molten metal so that a contact pressure between the casting mold and the molten metal is reduced.
However, none of these known methods has succeeded in satisfactorily preventing the occurrence of flux entrainment, because a macro flow of the molten metal is caused due to the moving magnetic field, or because the flow speed of the molten metal is increased in a position where the static magnetic field is small.